The present invention relates generally to devices and methods for measuring ambient aerosol distributions and properties, and more particularly, to a device and method capable of accurately measuring submicron aerosol size distributions with high time resolution and signal to noise ratio.
Aerosols refer to particles including solids, liquids and mixtures thereof suspended in a gas. Recent research has clearly demonstrated the significance of atmospheric aerosols on climate and air quality over regional and global scales. Atmospheric aerosols affect the climate by scattering and absorbing sunlight (direct effect) and by changing the microphysical structure, lifetime, and amount of clouds (indirect effect). The Intergovernmental Panel on Climate Change (IPCC) estimated that the direct and indirect effects of aerosols remain the most uncertain components in the climate system.
Prediction and mitigation of these effects require the knowledge of the spatial and temporal distributions, as well as the sources and governing processes, of the atmospheric aerosols. Among the aerosol properties needed for assessing climate impacts are size distributions and hygroscopicity. These variables determine climate forcing through their effects on light scattering and cloud formation. Particle size is also an important determinant for the study of penetration into bronchial airways and associated health effects.
Understanding and predicting the effects of aerosols on climate and air quality requires detailed characterization of ambient aerosol distributions and properties. However, due to its short lifetime, the spatial and temporal distributions of atmospheric aerosol are highly inhomogeneous. The current incomplete understanding of ambient aerosol properties, coupled with changing aerosol sources and sinks, necessitates further intensive field projects involving aircraft-based measurements. The advantages of aircraft-based measurements are obvious. These advantages include the ability to characterize 3-dimensional spatial distributions of aerosols and the ability to sample a large spatial domain within a short time. However, aircraft-base measurements are often compromised by the poor time resolution of current aerosol measurement techniques.
Conventional instruments for aerosol size distribution measurements include optical particle counter (OPC) systems and scanning mobility particle sizer (SMPS) systems, whose major component is a differential mobility analyzer (DMA). Conventional SMPS systems typically have time resolutions of about 50 seconds. The low speed of a conventional SMPS system of the prior art is a result of the system's sequential measurement method. Inside a DMA, charged aerosols migrate across flow streamlines under the influence of an electric field, and only aerosols of one size are selected and measured at one time. To obtain a complete submicron aerosol size distribution, which ranges approximately from 5 to 1000 nm, the electrical field inside the DMA has to be scanned through a wide dynamic range, and as many as 50 measurements of aerosols with different sizes are required. This takes about 50 seconds. Moreover, since only a small fraction of the aerosol is selected and measured, the counting rate of a SMPS system is very poor, which further slows down the measurements.
Even when efficiently automated, the time delay for completing a full measurement with a conventional DMA has its drawbacks. For example, a typical research aircraft travels several kilometers within a 50-second time period. Thus, accurate measurements of highly localized aerosol distributions, such as aerosols in the vicinity of a particular cloud formation or inside a pollution plume, are impossible since the aircraft will have traveled away from the localized aerosol source between the time the measurement begins to the time the measurement ends. Furthermore, due to the low sampling rate of DMA systems, measurements in clean environments are further restricted by the time required to obtain statistically significant numbers. Moreover, current SMPS systems are inadequate for detecting or measuring fast-changing size distributions of aerosol particles emitted by diesel engines and vehicles.
Other conventional aerosol measurement devices also have drawbacks. For example, conventional optical particle counters using light scattering techniques are generally only effective for measuring particle sizes greater than 200 nm. Aerosol hygroscopic property measurements, which are made using a tandem differential mobility analyzer (TDMA) system, also suffer from low time resolution and are rarely deployed in airborne studies.
Accordingly, it would be desirable to provide a measurement device which improves upon conventional DMA systems by simultaneously detecting aerosols of different sizes, thereby significantly improving measurement speed.